Satellites utilized for relaying broadcast signals and for other purposes are placed in geosynchronous orbit about the Earth so that they may be tracked with transmitting and receiving antennas that remain stationary. Such satellites must be located in an orbit about the Earth having a radius of 6.611 Earth radii to remain in a stationary position with respect to the Earth's surface. The plane of the orbit must be coplanar with the equatorial plane of the Earth to prevent apparent declination change. The period of orbital rotation of a satellite so situated is equal to the period of Earth rotation and its position as viewed from earth is constant.
Present recommended coverage guidelines suggest that such satellites should be located in an equatorial orbit between 70.degree. west longitude and 135.degree. west longitude for continential United States coverage. Various geosynchronous satellites utilized for United States broadcasting purposes are accordingly found on the arc described as the locus of points at 6.611 Earth radii from the center of the Earth, in the Earth's equatorial plane, and from approximately 70.degree. west longitude through approximately 135.degree. west longitude. Thus, Satcom F4, whose transponders relay various television network programming, is presently (in 1985) located at 83.degree. west longitude on this arc, while Galaxy 1, utilized for certain "super channels" and other television programming, is presently (in 1985) located at 134.degree. west longitude.
Those who transmit signals to or receive programs from such satellites or otherwise have reason to aim devices at such satellites frequently wish to reposition their antennas or devices toward various satellites to receive various programs or for other reasons. Existing antenna mounts allow such repositioning but require a compromise between accuracy and ease and speed of positioning.
In a first general group of existing mounts are antenna mounts which allow the antenna to be rotated about two axes. Two-axis mounts are advantageous because they may theoretically be pointed in any direction in the sky and accurately positioned by adjusting the rotation of the antenna about each axis. The two-axis systems are characterized by the orientation of the lower-most axis with respect to the ground. A two-axis system having its lower axis perpendicular to the ground is referred to as "elevation-over-azimuth" and one that has its lower axis parallel to the ground is referred to as an "x-y system." A system which has its lower axis parallel to the Earth's axis of rotation is referred to an "hour angle-declination" or "polar" mount because the left-right rotation of the antenna is about its hour angle axis (an axis parallel to the earth's polar axis) while the up and down rotation is about the antenna's declination axis.
An x-y mount is structurally less complicated than an elevation-over-azimuth mount because the ground and foundation provide direct support for the lower-most, horizontal elevational axis of the antenna. By contrast, the lower-most axis of the elevation-over-azimuth mount is perpendicular to the ground, requiring horizontally oriented structure to be located in the upper portion of the mount to support the elevational axis.
A typical x-y axis configuration places the x axis through the rear two feet of the mount and rotation about that axis may be adjusted with an adjustable front third foot. The rear two feet support a first, upper point of rotation for the y axis and the front foot supports the other point of rotation for the y axis. An adjustable structure between either of the two rear feet and the antenna allow the antenna to be adjusted about the y axis.
The hour angle axis in the polar mount is aligned parallel with the Earth's axis or inclined in a north-south direction from local horizontal at an angle equal to the site latitude. It is possible by rotating the antenna about the axis of such a system at one revolution per day to keep the antenna line of sight fixed at a point on the celestial sphere. Many astronomical telescopes have the polar axis configuration because of this characteristic. To point to a geosynchronous satellite in the equatorial plane, however, the antenna on such a mount must be depressed in declination because of the finite satellite orbital radius. The amount of declination required is a function of the satellite longitude, the site longitude and the site latitude.
Less complicated are single axis positioning systems. A single axis system which has the beam or elevational axis perpendicular to the axis of rotation may be considered to be an x-y mount whose lower axis is fixed after installation at a particular site. The system operates on the theory that rotation about an axis which is normal to the plane passing through the site and two geosynchronous satellites will aim the antenna at the two satellites with zero pointing error. The pointing errors for geosnychronous satellites between and beyond the two satellites are generally small. For example, if such a system is configured to point at Comstar 1.degree. at 128.degree. west longitude and Comstar 2 at 95.degree. west longitude, all satellites from 87.degree. through 135.degree. are within approximately 0.4.degree. of the antenna line of sight.
The single axis mount may be modified by "tilting" the antenna with respect to the rotational axis so that its line of sight forms a cone about the rotational axis. This configuration attempts to compensate for the offset of the antenna site with respect to the Earth's center.